1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a fusing unit of an image forming apparatus for fixing a transferred image on a printing medium, and a method for controlling a fusing pressure.
2. Description of the Related Art
Generally, electrophotographic image forming apparatuses, such as a printer, a photocopier and a facsimile, obtain a desired image by fixing on a printing medium a visible toner image transferred thereon with a fusing unit that has a heating roller and a pressing roller.
However, the heat and pressure required to fix the visible image on the printing medium differ according to a type of printing medium, particularly the thickness thereof. As a result, to effectively fuse in a manner appropriate for each type of printing medium, methods of controlling heat capacity transmitted to the printing medium and methods of controlling pressure applied to the printing medium have been developed.
To control the heat capacity applied to the printing medium, the time of transmitting the heat to the printing medium is controlled by adjusting a feeding velocity of the printing medium. When fixing an image on a printing medium having a great thickness and a high heat capacity, the printing medium is slowly fed so that heat can be evenly transmitted to the printing medium. By elongating the feeding time of the printing medium passing through the fusing unit, enough heat may be transmitted to the printing medium. According to this method, however, much time is required for image formation, thereby degrading printing efficiency.
The method of controlling the pressure applied to the printing medium according to the thickness of the printing medium may be achieved by a user manually operating a lever exposed out of the image forming apparatus. However, when the type of the printing medium is frequently changed or when a plurality of users share one image forming apparatus, it is cumbersome to operate the lever for every different type of printing medium or for each individual user to do so. Furthermore, according to this method, control of the pressure for subdivided types of the printing medium is difficult.
Accordingly, a need exists for an image forming apparatus having an improved fusing unit that controls the fusing pressure depending on the type of printing medium.